2012 DA14 is a classic example: It was discovered only 12 months ago, not by a space agency but by a Spanish dentist. It will come so close to earth that it will actually come between us and some of our satellites. It weighs either 130,000 or 180,000 tonnes (143,000 or 198,000 US tons), depending on which NASA website you trust, and is half as big as a sports stadium. For comparison, today's meteor over the Russian city of Chelyabinsk was only between 10 and 50 tonnes.

Oh, and even if we'd known it was coming? We would have been powerless to do anything, as no space agency is equipped to redirect the course of an asteroid.

These are the sorts of things that members of the B612 foundation, including former NASA scientists, worry about. Today, if ever, is their day. From an eye-opening post on their site:

…the fact that we knew ahead of time that 2012 DA14 is about to buzz by Earth is really only a matter of luck. Ninety nine percent of the time we are oblivious, simply because we have not mapped and tracked 99 percent of Near Earth Asteroids.

There are about a million asteroids in our solar system as large or larger than DA14. Of those, fewer than 10,000 have been identified and tracked.

Censuses of asteroids in our immediate neighborhood capture only a fraction of the objects that could

pose a threat. 2012 DA14 is in the <100 m category. NASA

Meteors strike earth five to ten times per year. Obviously, few are as dramatic as what exploded over Chelyabinsk. But sometimes we get unlucky. On June 30, 1908, something gigantic smacked into Siberia in what's known as the Tunguska Event. 770 square miles (2,000 square kilometers) of forest were flattened. If the object-either a meteoroid or comet-had struck a city instead, it would have wiped it out completely. The devastated region, note Ed Lu of B612 and Martin Rees, Britain's Astronomer Royal, was about the size of the San Francisco Bay Area.

And, they say, there is an estimated 30% chance of another such rock hitting the planet this century. The problem: unless we spot such rocks years in advance, we can't deflect them in time. Which is why B612 is trying to build a space telescope to spot and map all the asteroids big enough to pose a serious threat.

Editor's note: This story by Christopher Mims originally appeared on Quartz.